Combine raspberries, sugar, and lemon juice in a saucepan. Heat until bubbling, about 5-7 minutes. Strain and return juice, adding cornstarch to thicken.

Preheat oven to 325°F. Cream butter and sugar, then add egg whites. Combine flour and baking powder, folding in with sour cream and lemon zest. Divide into pans and bake for 40-42 minutes.

Boil water and dissolve sugar, then let it cool completely.

Beat butter, add powdered sugar, then melted white chocolate and cream. Mix until fluffy.

Brush cake layers with simple syrup, add raspberry filling and frosting between layers. Crumb coat and chill for 30 minutes.

Apply final frosting layer, decorate if desired, and chill again before serving.